Process of incorporating a refractory metal oxide in a metal and product resulting therefrom



Sept 29, 1964' G. B. ALEXANDER ETA 3 1 PROCESS OF INCORPORATING AREFRACTORS F METAL OXIDE IN A METAL AND PRODUCT RESULTING THEREFROMFlled Dec. 7, 1959 5 Sheets-Sheet l FIG.2

GLOBAR HEATING ELEMENT THERMOCOUPI-E 4C4 5"INCONEL TUBE WELL b I Q I1-)- HYDROGEN WATER AND IN HYDROGEN OUT I r I ,l l l s O-RING SEAL I I AE FIG. I

26,33? NICKEL 'NITRATE COLLOIDAL THoR|A AMMONIUM cA RBoNATE "PUMPINVENTORS:

GUY B. ALEXANDER WILLIAM H. PASFIELD PAUL C. YATES ATTORNEY TO FILTERSept. 29, 1964 G. B. ALEXANDER ETAL 3,150,443

PROCESS OF INCORPORATING A REFRACTORY METAL OXIDE IN A METAL AND PRODUCTRESULTING THEREFROM Filed Dec. 7, 1959 5 Sheets-Sheet 2 COOLING WATERHYDROGEN AND 0-!- WATER OUT HYDROGEN GLOBAR HEATING ELEMENT ---FIREBRICK SCRE EN INVENTORS Fl G 3 GUY B ALEXANDER WILLIAM H. PASFIELD PAULC. YATES ATTORNEY p 1964 G. B. ALEXANDER ETAL 3, 5 43 PROCESS OFINCORPORATING A REFRACTORY METAL OXIDE IN A METAL AND PRODUCT RESULTINGTHEREFRQM Filed Dec. 7, 1959 5 Sheets-Sheet 3 FIG.4

GRAIN BOUNDARY PARTICLE A REGION B REGlON A DIRECTION REG'ON 5 OFEXTRUSION REGION A INVENTORS GUY B ALEXANDER l WILLIAM H. PASFIELD 11-345 3 PAUL c. YATES LONGITUDINAL 20' EXTRUDED SECTION .7246. EMATTORNEY Sept. 1954 G. B. ALEXANDER ETAL 3,150,443

5 OF INCQRPORATING A REFRACTO METAL OXIDE A METAL AND PRODUCT RESULTINHEREFR PROCES IN Filed Dec. 7, 1959 5 ts-Sheet 4 INVENTORS LEXANDER GUYB A WILLIAM H. PASFIELD PAUL c. YATES ATTORNEY Sept. 29, 1964 G. B.ALEXANDER E P CORPORATING A REFRACTORY METAL OXIDE ODUCT RESULTIN FIG.7

INV GUY B ALEXANDER WlLLlAM H. PASFIELD PAUL C. YATES 4 ZEMW UnitedStates Patent 3,150,443 PRGCESS 0F INCORPORATING A REFRACTORY METAL 0EIN A METAL AND PRODUCT RESULTING THEREFROM Guy B. Alexander, BrandywineHundred, Del., William H. Pastield, Sayville, N.Y., and Paul C. Yates,Brandywine Hundred, Del., assignors to E. I. du Pont de Nemours andCompany, Wilmington, DeL, a corporation of Delaware Filed Dec. 7, 1959,Ser. No. 857,726 13 Claims. (Cl. 29--182.5)

This invention is concerned with improving the hightemperature servicecharacteristics of metals whose oxides have a free energy of formation(AF) at 27 C. of from 30 to 105 kilocalories per gram atom of oxygen(kcal./ gm. at. 0) and of alloys of these metals. The improvement isaccomplished by incorporating in the metals or alloys a particulatemetal dispersion of submicron-sized particles of a refractory metaloxide.

More particularly the invention is directed to processes in which (a) ahydrous, oxygen-containing compound of a metal having an oxide with a APat 27 C. of from 30 to 105 kcal./gm. at. 0 is deposited together withsubstantially discrete, submicron particles of a metaloxygen compoundwhich when heated to constant weight at 1500 C. is a refractory oxidehaving a melting point above 1000 C. and a AP at 1000 C. above 60kilocalories per gram atom of oxygen, (b) thereafter the hydrous,oxygen-containing compound is reduced to the corresponding metal whilemaintaining a temperature throughout the entire mass below the sinteringtemperature of the metal, whereby a powdered product is obtained, (0)the powdered product is sintered at a temperature below the meltingpoint of the metal until its surface area is less than about squaremeters per gram, and (d) the sintered powder is mixed with a powderedmetal.

The invention is further particularly directed to pulverulent productscomprising a particulate dispersion of submicron refractory oxideparticles of the above-mentioned type in a metal of the class to beimproved, said particulate dispersion having a surface area less than 10square meters per gram (m. /g.), mixed with a powdered metal having ahardness less than that of the refractory-containing metal, the AF ofthe refractory oxide at 1000" C. being at least the valuecalculated'from the expression 52+.016M, where M is the melting point,in "Kelvin, of the metal in the dispersion.

The invention is still further directed to sintered, solid metalcompositions in which there are volumes of a metal having substantiallyuniformly dispersed therein discrete, submicron particles of arefractory metal oxide having a AP at 1000 C. which is more than 60kcaL/gm. at. 0 and at least the value calculated from the expression52+.0l6M, where M is the melting point, in degrees Kelvin, of the metal,the said volumes being intermingled with other volumes of metal and theentire metal composition having an apparent density which is from 60 to100% of the absolute density.

The invention is additionally directed to solid, annealed metalcompositions of the type described, in which there are (a) volumes of ametal, the grains of which are in contact with discrete, submicronparticles of a refractory metal oxide having a AP at 1000 C. which ismore ice than 60 kcaL/ gm. atom 0 and at least the value calculated fromthe expression 52+.016M, where M is as above described, and (b) othervolumes of the same metal in which the metal grains are not in contactwith said refractory oxide particles, the average grain size in thevolumes (b) being at least two-fold the average grain size in volumes(a).

In the drawings FIGURE 1 illustrates operation of the initial step of aprocess of the invention,

FIGURE 2 shows one way to effect the reduction step of the process,

FIGURE 3 shows another embodiment of the reduction step,

FIGURE 4 is a line drawing prepared from an electronmicrograph of asolid product of the invention, showing the heterogeneous structure,

FIGURE 5 is also a line drawing prepared from an electronrnicrograph ofa solid product, in longitudinal section, showing the effect ofhot-working by extrusion,

FIGURE 6 is a fanciful representation of a powder product of theinvention, showing a mixture of filled and unfilled metal, and

FIGURE 7 is a fanciful representation of a pressed compact of a powderof FIGURE 6, before hot-working.

Attempts have already been made to incorporate refractory particles intosuch metals as copper in the hope that such inclusions might impartgreater strength to the metals, especially at elevated temperatures.Obtaining an adequate degree of dispersions has been a problem, however,and it has not hitherto been know how to incorporate discrete, veryfinely divided refractory particles into many other metals, such asiron, cobalt, and nickel and their alloys. Generally, oxide occulsionsin metals have been viewed with disfavor, and various means, such asscavenging with active metals, have been adopted to get rid of them.

Methods have been proposed for combining such finely divided powders asaerogels with finely divided metals, using the techniques of poweredmetallurgy, but have been unsuccessful as a way to improve thehigh-temperature properties of metals. In such methods the aerogel ismixed with the solid metal, and the whole mass is subjected to very highpressures. Under the circumstances, the ultimate particles in theaerogel structure are forced into intimate contact, producing a denselyaggregated structure which cannot be broken down and redispersed whenthe resulting compact is worked, either hot or cold. Thus, thecompositions prepared by this technique consist of metal having verycoarse oxide particles dispersed therein, the oxide particles being inthe 10 to micron range.

In our parent application Serial No. 694,086 filed November 4, 1957, nowPatent No. 3,019,103 we have described novel processes and compositionsin which the just-mentioned difficulties of the prior art have beenovercome. Very substantial improvements in the high-temperature servicecharacteristics of the products are achieved. When, as thereindescribed, the submicron refractory oxide particles are dispersedsubstantially uniformly throughout the entire metal mass the productsare very hard. While this is desirable in many situations, it does makethe metal difiicult to work by such common techniques as machining andforging.

Now according to the present invention it has been found that improvedhigh-temperature service characteristics can be achieved and adequateductility for easy workability can be retained by dispersing thesubmicron particles of refractory oxide filler in metal as in the parentapplication, and then mixing the filled metal, in powder form, with ametal powder unmodified with filler particles, the refractory oxidefiller and the filled and unfilled metal being so selected that the AFof the refractory oxide is at least the value calculated from theexpression 52+.0l6M where M is the melting point of the metal in K. Suchmixed powders can be compacted, sintered, and hot-worked to upwards of90% of the theoretical density to give solid metal products in whichthere are volumes of refractory-filled metal intermingled with othervolumes of unfilled metal. The latter products can be annealed until themetal grains in contact with the refractory particles have the samechemical composition as the metal grains not in contact with refractoryparticles, in which case the latter grains are at least twofold largerthan the former and are less hard. These sintered and the annealedproducts are surprisingly ductile, while retaining improvedhigh-temperature properties. In the processes for making the novelblended powders and the subsequent solid products unique and unexpectedfreedom from excessive and unwanted oxidation of the filled metal isachieved by sintering the filled metal particles until their surfacearea is below about square meters per gram before exposing them to airor other reactive gases.

THE FILLER In describing this invention the dispersed refractoryparticles will sometimes be referred to as the filler. The word filleris not used to mean an inert extender or diluent; rather, it means anessential constituent of the novel compositions which contributes newand unexpected properties to the metalliferous product. Hence the filleris an active ingredient.

In processes of this invention, a relatively non-reducible oxide isselected as the filler, that is, an oxide which is not reduced to thecorresponding metal by hydrogen, or by the metal in which it isembedded, at temperatures below 1000 C. Such fillers have a AF at 1000"C. of more than 60 kilogram calories per gram atom of oxygen in theoxide. The oxide itself can be used as the starting material or it canbe formed during the process by heating another metal-oxygen-containingmaterial.

The metal-oxygen-containing material can, for example, be selected fromthe group consisting of oxides, carbonates, oxalates, and, in general,compounds which, after heating to constant weight at 1500 C., arerefractory metal oxides. The ultimate oxide must have a melting pointabove that of the metal in which it is being used. Oxides with meltingpoints above 1000 C. are preferred. A material with a melting point inthis range is referred to as refractorythat is, difficult to fuse.Filler particles which melt or sinter at lower temperatures becomeaggregated.

The filler can be mixed oxide, particularly one in which each oxideconforms to the melting point and AF above stated. Thus, magnesiumsilicate, MgSiO is a mixed oxide of MgO and SiO Each of these oxides canbe used separately; also, their products of reaction with each other areuseful. The filler, accordingly, is a single metal oxide or a reactionproduct of two or more oxides, also, two or more separate oxides can beused as the filler. The term metal oxide filler broadly includesspinels, such as MgAl O and ZnAl O metal carbonates, such as BaCO metalaluminates, metal silicates such as magnesium silicate and zircon, metaltitanates, metal vanadates, metal chromites, and metal zirconates. Withspecific reference to silicates, for example, one can use complexstructures, such as sodium aluminum silicate, calcium aluminum silicate,calcium magnesium silicate, calcium chromium silicate, and calciumsilicate titanate.

Oxide AF at; Oxide AF at 1000 C 1000 C While any of the above-mentionedrefractory metal oxides will have utility, there is a correlationbetween the AF of the refractory and the melting point of the metal inwhich it is to be used. Thus, the AF of the refractory should be atleast the value calculated from the expression 52+.016M, where M is themelting point of the metal in which it is to be used. It will beunderstood that the melting point referred to is that of the metal as itexists in the final product. Thus, if the final metal is an alloy of twoor more metals, M will be the melting point of the alloy. Moreover, inthe blended powders or sintered compacts the filled metal can have onemelting point and the unfilled or ductile metal another; again, it isthe melting point of the final product which is to be considered.

It will be seen that as the melting point of the metal increases, the AFof the refractory to be used also increases. For example, the AF ofzirconia, 100, is too low to permit it to be used in tungsten metal(M.P.=

3370 C.) to get products of the invention; on the other hand, the AF ofthoria, 11-9, is well suited for use with molybdenum metal (M.P.=2620C.).

The filler oxide must be in a finely divided state. The substantiallydiscrete particles should have an average dimension in the size rangebelow 1 micron, preferably from 5 to 250 millimicrons, an especiallypreferred range being from 5 to millimicrons, with a minimum of 10millimicrons being even more preferred.

The particles should be dense and anhydrous for best results, butaggregates of smaller particles can be used, provided that the overallaggregate has the above-mentioned dimensions. Particles which aresubstantially spheroidal or cubical in shape are also preferred,although anisotropic particles such as fibers or platelets can be usedfor special efforts. Anisotropic particles produce metal compositions oflower ductility.

The size of a particle is given as an average dimension. For sphericalparticles all three dimensions are equal and the same as the average.For anisotropic particles the size is considered to be one third of thesum of the three particle dimension. For example, a fiber of asbestosmight be 500 millimicrons long but only 10 millimicrons wide and thick.The size of this particle is or 173 millimicrons, and hence within thepreferred limits of this invention.

Colloidal metal oxide aquasols are particularly useful as a means ofproviding the fillers in the desired finely divided form and hence arepreferred. For example, silica aquasols such as those described inBechtold et al. US. Patent 2,574,902, Alexander U.S. Patent 2,750,345,and Rule U.S. Patent 2,577,485 are suitable as starting materials.Zirconia sols are likewise useful. The art is familiar with titaniasols, and such sols as described by Weiser in Inorganic ColloidalChemistry, Volume 2, Hydrous Oxides and Hydroxides, for example, can beused. Also, the beryllia sols described on page 177 of this referencecan be used. Thoria aquasols can be prepared by calcining thoriumoxalate to 650 C. and dispersing the resulting colloidal thoria indilute acids.

Although they are less preferred, aerogels and reticulated powders canalso be used. For example, products described in Alexander et al. US.Patent 2,731,326 can be employed. In these instances it is necessarythat the aggregate structures be broken down to particles in the sizerange specified, for example, by colloid milling an aqueous slurry towhich a small amount of peptizing agent has been added.

Powders prepared by burning metal chlorides, as, for example, by burningsilicon tetrachloride, titanium tetrachloride, or zirconiumtetrachloride to produce a corresponding oxide, are also very useful ifthe oxides are obtained primarily as discrete, individual particles, oraggregated structures which can be dispersed to such particles.

Calcium oxide is a useful filler. Since this oxide is water soluble or,more accurately, Water reactive, one cannot obtain it as an aqueousdispersion in the colloidal state. In this instance, one can use aninsoluble calcium compound, such as the carbonate or oxalate, which, onheating, will decompose to the oxide. Thus, for example, particles offinely divided calcium carbonate can be coated with an oxide of themetal in which it is to be dispersed, e.g., hydrous iron oxide, bytreating a dispersion of finely divided calcium carbonate with a baseand a salt of the metal, e.g., ferric nitrate and sodium carbonate. Onheating the precipitate and reducing, a dispersion of calcium oxide iniron is obtained. Similarly, one can obtain dispersions of barium oxide,strontium oxide, or magnesia in the metal being treated.

THE METAL TO BE FILLED The metal in which the refractory oxide is to beincorporated is selected from the group consisting of metals whoseoxides have a free energy of formation at 27 C. of from 30 to 105kcal./gm. at 0. These metals, and the free energies of formation oftheir oxides, are as follows:

of these metals can, of course, be used.

COATING THE FILLER In processes of preparing the filled metal particles,a relatively large volume of the metal oxide, hydroxide, hydrous oxide,oxycarbonate, or hydroxycarbonate, or, in general, any compound of themetal wherein the metal is in an oxidized state, is formed as a coatingaround the refractory oxide filler. This coating can be a compound ofsingle metal, or it can contain two or more metals. For example, thehydrous oxides of both nickel and cobalt can be deposited around afiller. In the latter case, an alloy of nickel and cobalt is produceddirectly, by reducing with hydrogen.

In a similar manner, alloys of any metals which form oxides that can bereduced with hydrogen, can be prepared. Thus, alloys of iron, cobalt,nickel, copper, molybdenum, tungsten, chromium and rhenium can beprepared by codepositing oxides of two or more of the selected metals onthe filler particles and subsequently reducing.

To produce such a hydrous, oxygen-containing composition one canprecipitate it from a soluble salt, pref- Mixtures 6 erably a metalnitrate, although metal chlorides, sulfates, and acetates can be used.Ferric nitrate, cobalt nitrate, and nickel nitrate are among thepreferred starting materials.

The precipitation can be conveniently accomplished by adding a suitablesoluble metal salt to an aqueous alkaline solution containing the fillerparticles, while maintaining the pH above 7. A good way to do this is toadd, simultaneously but separately, a concentrated, aqueous solution ofthe soluble metal salt, a colloidal aquasol containing the fillerparticles, and an alkali such as sodium hydroxide, to a heel of water.Alternatively, a dispersion containing the filler products can be usedas a heel, and the metal salt solution and alkali added simultaneouslybut separately thereto.

More broadly, in depositing the compound of a metal in an oxidized stateupon the filler, one can react any soluble salt of these metals with abasic material. Hydroxides such as NaOH, KOH, or ammonia, or carbonatessuch as (NH CO Na CO or K 00 can be used. Thus, the metal compounddeposited can be an oxide, hydroxide, hydrous oxide, oxycarboante, or ingeneral, a compound which, on heating, will decompose to the oxide.

During the coating process certain precautions are preferably observed.It is preferred not to coagulate or gel the colloid. Coagulation andgelation are avoided by simultaneously adding the filler and the metalsalt solution to a heel.

It is preferred that the filler particles be completely surrounded withthe reducible oxides or hydrous oxides such as those of iron, cobalt ornickel, so that when reduction occurs later in the process, aggregationand coalescence of the filler particles is avoided. In other words, itis preferred that the ultimate particles of the filler be not incontact, one with another, in the coprecipitated product. Anothercondition which is important during the preparation to insure thiscondition is to use vigorous mixing and agitation.

Having deposited the hydrous oxygen compound of the metal, such as iron,cobalt or nickel, on the filler, it is then desirable to remove thesalts formed during the reaction, by washing. Ordinarily, one uses analkali such as sodium hydroxide, potassium hydroxide, lithium hydroxide,ammonium or tetramethylammonium hydroxide in the deposition of thecompound. As a result, salts like sodium nitrate, ammonium nitrate orpotassium nitrate may be formed. These should be removed, sinceotherwise they may appear in the final product. One of the advantages ofusing the nitrate salts in combination with aqueous ammonia is thatammonium nitrate is volatile, and therefore is easily removed from theproduct. However, the tendency of many metals, such as cobalt andnickel, particularly, to form amine complexes, is a complicatingreaction in this case. By carefully controlling the pH duringcoprecipitation, these side reactions can be avoided.

Having essentially removed the soluble, non-volatile salts by washing,the product is then dried at a temperature above C. Alternatively, theproduct can be dried, and the dry material suspended in water to removethe soluble salts, and thereafter the product redried.

PROPORTIONS OF DEPOSITED METAL COM- POUND AND FILLER The relative amountof oxidized metal compound coating which is deposited on the fillerparticles depends somewhat on the end use to which the product is to beput, but is generally such that the end product contains an amount offiller in the range from 1 to 35% by volume and preferably from 5 to20%. At loadings below 1% the filled metal is too soft, and at very highloadings the powders are difficult to work with because there isinsufiicient metal phase to hold the aggregate structure together.

Volume loadings as high as 50%, that is, one volume of oxide for eachvolume of metal present, can be successfully used, but such products areoften pyrophoric. Even heating to a temperature approaching the meltingpoint of the metal after reduction does not completely eliminate thisproblem. Also, the filler particles in such products tend to coalesce toform large, hard aggregates during the reduction step. This tendency canbe reduced by increasing the particle size of the filler, say to 100millimicrons or even larger. The problems just discussed are minimizedas the volume loading is reduced. In the range of 40 to 50 volumepercent of filler, it is advisable to protect the modified metal in aninert atmosphere (hydrogen, argon or nitrogen) until the material isblended with unmodified metal and compacted. Even at 10 volume percent,one often has difiiculty sintering the modified metal mass sufficientlythat it can be handled in air.

REDUCING THE PRECIPITATE CONTAINING THE FILLER Having deposited thecompound of metal in oxidized state around the filler particles anddried the product, the next step is to reduce the coating to the metal.This can be conveniently done by subjecting the coated particles to areducing agent, such as a stream of hydrogen at a somewhat elevatedtemperature. However, the temperature throughout the entire mass mustnot be allowed to exceed the sintering temperature of the fillerparticles. One way to accomplish this is to place the product in afurnace at controlled temperature, and add hydrogen gas slowly. Thus,the reduction reaction will not proceed so rapidly that large amounts ofheat are liberated and the temperature in the furnace is increased.

Hydrogen to be used in the reduction can be diluted with an inert gassuch as argon, or in some cases nitrogen, to reduce the rate of reactionand avoid hot spots. In this way the heat of reaction will be carriedaway in the gas stream. Alternatively, the temperature in the furnacecan be slowly raised into the range of 500 to 700 C. while maintaining aflow of hydrogen over the product to be reduced.

In addition to, or instead of, hydrogen, carbon monoxide can be used asthe reducing agent, particularly at elevated temperatures, as well asmethane or other hydrocarbon gases. In any case, it is important thatthe temperature during reduction be controlled, not only to avoidpremature sintering as above mentioned, but also so that excessivereaction will not occur between the reducible compound (such as iron,cobalt or nickel oxide) and the filler oxide before the reduciblecompound is reduced.

Reduction should be continued until the coating is essentiallycompletely reduced. When the reaction is nearing completion, it ispreferred to raise the temperature to complete the reduction reaction,but care must be taken not to exceed the melting point of the reducedmetal. Reduction should be carried out until the oxygen content of themass is substantially reduced to zero, exclusive of the oxygen of theoxide filler material. In any case, the oxygen content of the filledmetal, exclusive of the oxygen in the filler, should be in the rangefrom to 2% and preferably from 0 to 0.05%, based on the weight of thefilled metal.

One way of estimating the oxygen content is to measure the change inweight of a product on treatment with dry, oxygen-free hydrogen at 1300C. Products which show a change in weight of only from 0.0 to 0.05%under this condition are preferred.

After the reduction reaction is complete, the resulting powder issometimes pyrophoric. Therefore, it is preferred to cool and store themass in an inert atmosphere such as argon, and further to blend andcompact the mass to reduce surface area in the absence of oxygen ornitrogen.

It is particularly desirable that the filled metal powder be stored inan inert atmosphere such as argon if its surface area is greater than .1square meter per gram. The atmosphere should be essentially free ofoxygen, water vapor, nitrogen, sulfur, and any other elements orcompounds which are reactive with the metal powder.

An alternate way of reducing the metal in contact with the refractoryfiller is to subject the coated particles to a metal reducing agent in afused salt bath. The compound coated refractory oxide particles aredispersed in the molten salt and the reducing metal is added whilemaintaining the temperature of the molten salt in the range of 400 to1200 C.

The fused salt bath is merely a medium whereby to effect contact of thereducing agent and the metal compound under conditions which will notaffect the disposition of the compound with respect to the refractoryparticles. It can comprise any suitable salt or mixture of salts havingthe necessary stability, fusion point, and the like.

Suitable fused salt baths can comprise halides of metals selected fromgroups I and 11a of the periodic table.

In general, the chlorides and fluorides are preferred halides. Bromidesor iodides can be used, although their stability at elevatedtemperatures is frequently insufficient. Chlorides are especiallypreferred. Thus, among the preferred salts are calcium chloride, sodiumchloride, potassium chloride, barium chloride, strontium chloride, andlithium chloride and fluoride.

The fused salt bath will usually be operated under a blanket of eitheran inert gas or a reducing gas. Such gases as helium, argon hydrogen orhydrocarbon gases can be used in this capacity.

The temperature of the reduction can be varied considerably, dependingupon the combination of fused salt and reducing metal selected. Ingeneral, the temperature of reduction will be between 400 and 1200 C. Itis usually preferred to select a reduction temperature at which thereducing metal, as well as the fused salt, is present in a molten state.Usually the operating temperature will also be below the boiling pointof the reducing metal employed.

The operating temperature of the reduction bath must also be below themelting point of the metal coating to be produced on the refractoryfiller. For example, if a tungsten compound is being reduced uponparticles of thoria, reduction temperature as high as 1200 C. can beemployed.

However, if a compound of copper, or of a coppercontaining alloy havinga low melting point, is being reduced, the reduction temperature shouldbe maintained below that of the melting point of the copper or thealloy.

The reducing metal is selected from the group consisting of alkali andalkaline earth metals. Thus, the metal can be lithium, sodium,potassium, rubidium, caesium, beryllium, magnesium, calcium, strontium,or barium.

It is preferred to use a reducing metal which has a low solubility inthe solid state with respect to the metal of the coating on therefractory oxide particles; otherwise, one will get undesirable alloyingof the reducing metal with the metal formed by the reduction. For thisreason, calcium and sodium are suitable for reducing compounds of suchmetals as iron, cobalt, nickel, chromium, or tungsten, while magnesiumand sodium are useful in reducing titanium.

It is preferable to use a temperature of reduction at which thereduction reaction proceeds at a rapid rate. For reducing cobalt, iron,and nickel compounds, temperatures in the range of 600 to 800 C. aresuitable. With compounds of metals such as chromium, titanium, andniobium, temperatures in the range of 850 to 1000 C. are used.

Completion of the reduction reaction can be determined by taking samplesfrom the melt, separating the product from the fused salt, and analyzingfor oxygen by ordinary analytical procedures such as vacuum fusion.

The reduction is continued until the oxygen content of the mass issubstantially reduced to zero, exclusive of the oxygen of the oxiderefractory material. In any case, the oxygen content of the product,exclusive of the oxygen in the refractory, should be in the range offrom to 2 percent and preferably from 0 to 0.05%, based on the weight ofthe product.

The reduced product is present as a suspension in the fused salt bath.It can be separated therefrom by the techniques ordinarily used forremoving suspended materials from liquids. Gravitational methods such assettling, centrifuging, decanting and the like can be used, or theproduct can be filtered off. Alternatively, the bath can be cooled andthe fused salt dissolved in a suitable solvent such as dilute aqueousnitric acid or acetic acid.

If a considerable excess of reducing metal is used in the reductionstep, it may be necessary to use a solvent less reactive than Water forthe isolation procedure. In such a case, methyl or ethyl alcohol issatisfactory. The presence of a small amount of acid in the isolationsolvent will dissolve any insoluble oxides formed by reaction betweenthe reducing metal and the oxygen content of the coating being reduced.After filtering off the reduced metal powder, it can be dried to free itof residual solvent.

It will be understood that irrespective of which of the above-mentionedreduction methods is used one can mix a dried, refractory-filled metaloxide and an oxide of the metal which is to be the matrix of the finalproduct and coreduce the mixture to get solid metal products of theinvention.

THE REFRACTORY-FILLED METAL POWDER The refractory-filled metal powderobtained as just described can be compacted and sintered directly touseful metal products as disclosed and claimed in our abovementionedparent application Serial No. 694,086. To get the heterogeneousisland-structured products of the present invention the compacting iscarried out in the presence of unfilled metal powders. Thus,compositions of the parent application can be used as a masterbatc inmaking the powder metallurgical products of the pres ent case. Forexample, iron containing zirconia can be used as a masterbatch andalloying agent in other metallurgical compositions, and can further bediluted with untreated iron as a finely divided powder, or can also beused as an alloying agent for other metals, such as, for example,copper, molybdenum, nickel, and tungsten. Compacting to a dense body canbe effected after mixing the iron containing the Zirconia with thefinely divided powders of the other metals.

In a similar manner, masterbatches of cobalt or nickel containing thedesired fillers can be used to prepare various alloys. For example, animproved type of nickelcopper alloy having island structure can beprepared by dispersing zirconia in nickel and thereafter alloying thenickel with copper by powder metallurgical techniques. The oxidationresistance of products made in this manner is also improved. Thus, suchmaterials are more oxidation resistant than the unmodified metals, theextent of improvements being related to the amount of dilution withunmodified metal.

In still another specific example, a nickel-molybdenum alloy can beprepared from a masterbatch of nickelthoria, nickel-alumina ornickel-calcia. In making masterbatches, higher loadings of oxides areused, volume loadings in the range from 2 to 20% being preferred. Inthis way, a modified nickel powder of very high hardness is produced.This modified nickel powder is then blended with a nickel-molybdenum,oxide-free powder, rich in molybdenum. Blending is accomplished bytumbling the powders (which are preferably small enough to pass througha 325-mesh screen) in a conical or cubical container for several hours.The blended powder is compacted to a non-porous mass, i.e., having adensity at least 95% of absolute density. Thereafter, the composition isheat-soaked and worked, for example, at 1300 C., in order to diffuse themolybdenum throughout the nickel. The resulting composition, containingnickel as a metallic component, is considerably more ductile than acomposition containing similar components but made directly. In makingalloys in this manner it is important to use powders which areessentially oxygen free, except for the oxygen in the filler.

It is believed that the reason for the above-mentioned improvement inductility is that the composition made by powder dilution from amasterbatch consists of two phases: one phase, the matrix, of ductile,unmodified, large-grained metal and another phase, the islands, ofextremely hard, fine-grained, highly reinforced metal. The unmodifiedphase gives the composition ductility and the hard phase (metal-l-oxide)gives it unusual strength at high temperatures. Thus, the advantage ofthe above dilution technique is that it produces a metal material ofhigher ductility and lower hardness, and at the same time giveshigh-temperature strength properties due to the presence of the oxidefiller.

The nature of the metallic component and refractory and the proportionsof these components in the filled metal powdershave been described atlength, above. Other factors important in the preferred blended powderswill now be discussed.

The particle size of the filled metal to be used in the powder blendingpreferably is kept relatively small, i.e., less than 500 microns, andcan be as small as 1 micron. minimizing the particle size can beaccomplished by minimizing sintering during reduction, i.e., by reducingat a minimum temperature. If the powder particles are too small theyhave a tendency to be pyrophoric; however, even such powders are usefulif handled in the absence of air, for example, in a clean, dry argon orhydrogen atmosphere. Powders like thoria-filled nickel orlanthana-filled iron can be handled in helium or hydrogen.

Powders for blending should in any event pass 50 mesh. Powders whichwill pass 325 mesh (44 microns) are desirable, and powders of particlessmaller than 10 microns are preferred. Size as used here refers to theaggre gate structure of the metal-metal oxide powders. Such powders areporous and may have internal surface area which is 10 or even times thesurface area which would be calculated from overall dimensions.

The grain size of the metal in the filled-metal particles ordinarily isless than 10 microns and preferably in the range of 0.5 to 3 microns.Grain size can be estimated by the calculation where G is the grain sizein microns, d is the diameter of the filler particles in microns, and fis the volume fraction of the filler. To illustrate, in analumina-filled chromium powder in which alumina particles of 0.3 micronare present to 15% by volume, the estimated grain size would be 0.7micron.

If it is desired to measure grain size directly, this can be done by anyconventional metallurgical technique, for example, as indicated inExample 1.

It is preferred that the filler-metal powder be reasonably free ofoxygen, nitrogen or any contaminant which may interfere with diffusionor with bonding at grain boundaries, or which may impart brittleness tothe metal of the ultimate product. Oxygen, for example, preferablyshould be less than 0.05%, exclusive of oxygen in the filler.

THE UNFILLED METAL POWDER FOR BLENDING The powder selected for blendingwith the masterbatch should be ductile. In the case of many metals, likechromium, this means that it must be essentially free of certaincontaminants, for example, oxygen, nitrogen, carbon,

3 l. boron, and sulfur. The ductility of this phase is particularlyimportant, since otherwise it will be difiicult to prepare dense,metallurgically sound metal after compacting and working the blendedpowders.

The powder preferably should be a metal or an alloy which, when inmassive form, forms a somewhat protective oxide film. In the finalproduct, the filler will improve the protective nature of the oxidefilm. Thus, for products of maximum corrosion resistance the unfilledmetal powder preferably will consist of chromium, nickel, cobalt, iron,titanium, manganese, vanadium, silicon, aluminum, magnesium, zinc,zirconium, niobium, copper, the rare earth metals, and alloys of any ofthese metals. In general, these are metals having a melting point in therange of from 200 to 2400 C. and which form oxides having a free energyof formation in the range from 30 to 140 kcal./ gm. atom of oxygen at 27C.

If the metal powders being blended are pyrophoric, they should beblanketed by an inert atmosphere during blending. This will preventoxidation, and hence undesired contamination.

The ductile metal powder may contain a relatively small amount offiller, as, for example, up to about 1% by volume. However, as theamount of filler increases, the ductility of the powder decreases, andhence filler contents greater than about 4% by volume in the ductilemetal phase are generally not desired.

The ductile metal powder should also have a large grain size, or becapable of recrystallizing to large grains on heating to a temperatureof 0.6 times its melting point in degrees absolute. If the potentialgrain size is calculated as above, it should be greater than microns andpreferably greater than 50 microns. If one has filler particles whichare 0.1 microns in size in the ductile metal phase, the preferredconcentration of the tiller particles is less than 0.25 volume percent.With filler particles of this size, i.e., 0.1 micron, volume loadings aslow as 0.02% will produce a beneficial effect. Thus, a preferred powderfor use as the ductile phase is one containing filler at such a loadingand of such size that the calculated grain size is in the range from 50to 500 microns. In such compositions, the tiller should be uniformlydispersed throughout the ductile metal phase, and should be less than 1micron in size. Such powders can be prepared using a process similar tothat used in preparing the refractory-filled metal powder, except that amuch smaller amount of filler is added.

In selecting ductile metal powders for blending, it is preferred thatthey be in the size range below 500 microns. More preferred powders arethose which are in the size range from 2 to 20 microns.

THE POWDER-BLENDING STEP Blending ratios of masterbatch to ductile metalcan vary over wide ranges. However, in most cases, from 0.1 to 10 partsof masterbatch, or filled metal powder, is blended with 1 part ofductile metal powder. In the more preferred case, from 0.3 to 2.5 partsby volume of masterbatch is blended with each part by volume of ductilemetal powder.

The processes of the invention include the step of blending arefractory-filled metal powder, prepared as above described, with aductile metal powder. This can be accomplished in several ways, as, forexample, by tumbling, ball milling or any other processes used in theart for mixing powders. Instead of blending metal powders, one can blendcompounds which are reducible to metals, such as a metal oxide,hydroxide, hydroxycarbonate, or carbonate. In the latter embodiment, anymetal compound which on heating and reduction can be converted to ametal can be used in place of the ductile metal powder. As an example, anickel-beryllia powder can be blended with nickel oxide, and the blendedpowders reduced in hydrogen at a temperature of 400 to 900 C.Conversely, a nickel hydroXide-beryllia masterbatch can be mixed withnickel hydroxide, and this mixture reduced after blending. However, theuse of a refractory-filled metal powder in the blending operation ispreferred.

COMPACTING THE POWDERS Following the blending steps, the entire mass ofblended, unfilled metal powder and filled metal powder masterbatch iscompacted. This can be done by ordinary powder metallurgical techniques,for example, by subjecting the product to high pressures, at ordinarytemperatures or preferably at temperatures equivalent to about twothirds of the absolute melting point of the ductile metal. In someinstances, it is desirable to heat the product during this pressingoperation to temperatures just slightly below the melting point.However, it is preferred that the metal be not melted.

The powder can be hydraulically pressed, hydrostatically pressed, orloaded in an evacuated container and subjected to hot forging.

A preferred combination for compacting is a refractoryfilled,high-melting metal powder blended with a powdered, low melting metal.With such a mixture, by hot pressing at a pressure in excess of thetensile strength of the low melting metal, dense compacts can be formed.This compaction is accomplished before homogenization of the metalfractions has occurred.

During compaction the metal powders should not be exposed to oxidizingconditions. In handling high surface area powders, it is preferred thatthe powders be completely free of exposure to oxygen until they havebeen compacted to maximum density.

STNTERING THE COMPACT The green compact formed as just described can besintered, as at temperatures up to of its melting point for up totwenty-four hours, to give it suflicient strength to hold togetherduring subsequent working operations. Preferably, the sintering iseffected in an inert atmosphere, such as argon or helium, or in areducing atmosphere such as clean, dry hydrogen. If the green compacthas sufiicient strength, the sintering step can be omitted, and anannealing step substituted after working.

WORKING THE COMPACT To obtain metal products of maximum density, and toachieve maximum bonding of the metal grains, the compacted body issubjected to intensive working, preferably at elevated temperatures. Theworking forces should be sufficient to effect plastic flow in themetals. Working should be continued until welding of the filler-metalgrains and ductile metal grains is substantially complete.

While working can be accomplished by such methods as swaging, forging,and rolling, it is especially preferred to effect working by extrudingthe above-mentioned compact through a die under extreme pressure. Afterextrusion, the product can, if desired, be further worked by swaging,forging or rolling.

Whichever method of working is selected, it is preferred that exposureto oxygen, nitrogen and water be avoided during the working step. Evensmall amounts of oxygen will form oxide skins on the metal powders.These skins increase the difficulty of welding the powders, hence theirformation is preferably avoided.

Because the refractory-filled powders are very hard, the forces requiredfor working are higher than for unmodified metal. In the case ofextrusion of a billet, the reduction in cross-sectional area preferablyis upwards of 90%. Welding of the metal grains thereupon becomes nearlycomplete.

Now while the worked products have greatly increased strength andusually have increased hardness, they nevertheless are relativelyductile. It is believed that the increased strength is due to the volumeof hard or reinforced metal containing the filler, in which the metalgrain size is less than 5 microns in average diameter and usually of theorder of l to 2 microns, whereas the ductility is due to other volumesof metal, in which the grain size is of the order of 10 to 100 micronsor even larger. The ratio of the volumes of small to large grains isapproximately equivalent to the ratio of filled to unfilled metal usedin any given preparation; therefore, control of these ratios givescontrol of ductility in the final product.

Because of the ductile phase, the product after working may have apattern or structure in the direction of working, as shown in FIGURE 5.This can be minimized or eliminated if desired by working the metal inseveral directions.

The final compacted and sintered mass of metal to be used as a materialof construction should have a density upwards of 95% of theoretical,preferably upwards of 99%.

ANNEALING THE SOLID METAL PRODUCTS Although as above noted the solidmetal products of this invention are heterogeneous in the sense that themetal grains in contact with the refractory filler particles are muchsmaller than the metal grains not in such contact, it is not desiredthat the metal originally associated with the filler remain separate anddistinct from the metal derived from the unfilled metal powder. On thecontrary, it is desirable to have chemical homogenization of these twometal phases, so that all metal areas present are substantially the sameexcept in regard to grain size.

The desired chemical homogenization is accomplished by annealing thecompacted powder blend. This step can be carried out before, after, orsimultaneously with the working step above described, in the latter caseby suitably selecting the temperature of hot-working. The annealingcauses diffusion of the metal phases into each other, so that bychemical analysis, the metal is shown to be of uniform compositionthroughout the mass of the solid, annealed product. For instance, if thefilled metal is nickel and the unfilled metal powder is chromium, themetal in the product after annealing is completed will be a uniformmixture, or alloy, of nickel and chromium. The size of the alloy grainsin contact with the filler particles will be smaller than the size ofthe alloy grains not in such contact.

The time and temperature of annealing will, of course, depend upon suchfactors as the metals used, their proportion, and the proportion ofrefractory particle in the metal. Since chemical and metallographictechniques are available for determining the extent to whichhomogenization has taken place during annealing, those skilled in theart will have no difficulty selecting suitable conditions for anyparticular system. In general, annealing can be done at temperatures inthe range of from 0.75 to 0.9 of the melting point in degrees absolute.

THE NOVEL COMPOSITIONS The novel compositions of the present inventioninclude the powder products comprising mixtures of refractoryfilledmetal particles and unfilled metal particles, and also the solid metalsof island structure which can be made by solidifying such metal powders.The latter metal products are ductile, resistant to creep at elevatedtemperature, and resistant to oxidation.

In the product characterizations set forth herein, descriptions willsometimes be given with particular reference to single-metalcompositions of the invention, but it will be apparent that thecharacterizations can also be applied to alloy products.

The filler particles present in the filled metal component of productsof this invention are dispersed throughout said component. Thisdispersion can be demonstrated using the electron microscope and replicatechniques wherein the surface of a metal piece is polished, a carbonlayer is deposited on the polished surface, and the metal is removed, asby dissolving in acid. An electronmicrograph of the remaining carbonfilm shows that the f4 filler particles are uniformly distributedthroughout the metal grains.

By uniformly dispersed is meant that there is uniform distribution ofthe refractory oxide particles within any single selected microscopicregion of treated metal, such regions being about 10 microns indiameter. Since treated metal powder is blended with untreated metalpowder and the mixture is compacted and sintered to dense metal, it isobvious that only those regions originating from particles of treatedmetal powder will contain a uniform distribution of the filler in themetal matrix.

The filler particles in compositions of the invention must be in thesize range of 5 to 1000 millimicrons, preferably from 5 to 500millimicrons, and still more preferably from 10 to 250 millimicrons. Thelatter class is particularly preferred, since the 10 millimicronparticles are considerably more resistant to coagulation or gelling thanare smaller particles, and thus are easier to maintain in a dispersedstate during processes of the invention than smaller particles. Thus,products containing filler particles in the size range of from 10 to 250millimicrons can be readily produced according to processes of thisinvention from colloidal dispersions.

In describing products of this invention an oxide filler particle isdefined as a single coherent mass of oxide surrounded by metal andseparated from other oxide mass by metal. The particles may beaggregates of smaller ultimate units which are joined together to form astructure, but, of course, the size of the aggregate must be in therange of 5 to 1000 millimicrons.

The particles of the filler in the filled-metal component aresubstantially completely surrounded by a metal coating which maintainsthem separate and discrete. The particles are thus isolated, and do notcome in contact one with another; thus, coalescence and sintering of thefiller material is prevented. In other words, the filledmetal componentcomprises a continuous phase of metal containing, dispersed therein, therefractory filler particles.

This aspect of the invention is illustrated by FIGURE 6 of the drawings,wherein the area encompassed by line 1 represents a refractory-filledmetal powder particle consisting of metal grains 2 in which there aredispersed the particles of refractory filler 3. It will be understoodthat the lines 6 within the powder particle represent the grainboundaries of the metal. FIGURE 6 illustrates a powder product of theinvention in that there are also present unfilled, powdered metalparticles, shown as encompassed by line 4 and made up of metal grains 5in which there are no dispersed refractory particles.

Metal compositions in which the filler is thoria, a rare earth oxide, ora mixture of oxides of the rare earth elements of the lanthanum andactinium series, magnesium oxide, or, to a lesser extent, calciumsilicate, have exceptional stability in elevated-temperature, long-comtinned tests such as stress rupture and creep tests. These materialsmaintain their properties to a considerably greater extent than metalsfilled with silica, for example, even when the initial hardness obtainedduring the processing operation is similar. The reason for thisimprovement is related to the free energy of formation of the filler.For this reason, preferred compositions of the invention for use at veryhigh temperatures, i.e., above 800 to 1000 C., comprise a dispersion, ina metal, of oxide particles having a free energy of formation asdetermined at 1000 C. per gram atom of oxygen atom in the oxide, of fromto 123 kcal. and preferably from to 123 kcal.

Actually, silica is a highly efficient filler for metal compositionswhich do not need to be heated above 600 to 700 C. during processing oruse. In the case of iron-molybdenum or nickel-molybdenum alloys, whichare made by blending molybdenum powder with powders of modified iron ormodified nickel, temperatures as 15 high as 1300 C. or slightly higherare often encountered during processing. In these cases, only the verystable oxides are effective as fillers, i.e., those with a very highfree energy of formation, such as the rare earth oxides or calcia.

This correlation between high-temperature service and the free energy offormation of the refractory filler can be generalized in terms of themelting point of the metal or alloy involved, since for high-temperatureservice metals of high melting point are used. Thus, it is a limitationupon the compositions of this invention that the free energy offormation of the particulate refractory (as measured at 1000 C.) be atleast 52+.016M, where M is the melting point in degrees Kelvin of themetal or alloy phase of the solid product.

Products of the invention can be characterized by the distance betweenthe filler particles in those metal volumes in which such particles arepresent. This distance is a variable which depends on both volumeloading and particle size. If the dispersed phase is a material ofuniform particle size and is dispersed homogeneously in a cubic packingpattern, the following expression relates the interparticle distance,i.e., the edge-to-edge distance Y, to the particle diameter d and thevolume fraction of the dispersed phase f:

itts-fl For products of this invention, the interparticle distance ascalculated by this expression is less than 1.0 micron and preferablyfrom 0.01 to 0.5 microns (10 to 500 millimicrons). In the most preferredproducts this range is 50 to 250 millimicrons, in those metal volumes inwhich such filler is present.

The finely divided filler particles in compositions of the inventioncauses the grain size of the metal in the vicinity of the filler to bemuch smaller than normally found. This small grain size persists evenafter annealing at temperatures in degrees absolute up to 0.8 times thatof the melting point of the products. A grain size below 10 microns, andeven below 2 microns is common for the products of this invention.Products which have filler particles in contact with metal grains in thesize range below 10 microns are preferred.

The solid metal products obtained by compacting, sintering and suitablyworking the blended powders as above described are a particularlypreferred embodiment of this invention. In the drawings, FIGURE 7represents the kind of structure existing in the pressed compact,wherein there are volumes or islands of filled metal 1 containing fillerparticles 3, these islands being interspersed throughout other volumesof unfilled metal particles 4. It will be seen that there are stillvoids '7 between the filled and unfilled metal particles; in otherwords, theoretical density is not achieved or closely approached merelyby compaction. Further working is required.

FIGURE is a line drawing prepared from an electronmicrograph showing theisland structure in a solid product of the invention which has beenhot-worked by extrusion to a density approaching theoreticalthat is,above about 90% of theoretical. FIGURE 4 is similarly a line drawingmade from an electronmicrograph, but in this figure the solid producthad not yet been extruded. It will be seen that in FIGURE 4 there is aregion of filled metal, B, in which the grain bounderies are very closetogether, i.e., the metal grains are very small, and a region ofunfilled metal, A, in which the grain boundaries are relatively farapart and the grains are relatively large. In FIGURE 5 regions A and Bare also present, but the former appears as islands in the latter.

Actually, while the term island structure is useful to convey theconcept of heterogeneous character present in solid products of thisinvention, it can be a misnomer in some instances. Thus, while therewill always be areas (or rather, volumes) of refractory-filled metalintermingled with areas (or volumes) of unfilled metal, it is immaterialwhich is the island and which the matrix. This will depend upon whichcomponent is present in predominant proportion, the minor componentusually being the islands, although as seen from a comparison of FIGURES4 and 5 the electronmicrographs may seem to indicate one structurebefore extrusion and the other after extrusion, even on the same sampleof product.

Because the products of the invention have volumes of metal containingno filler, they are more ductile than those compositions in which thefiller is uniformly dispersed throughout the composition. In thosevolumes in which there are no filler particles, the metal grains grow toa much larger size, i.e., 40 microns and larger.

The size and shape of the filled and unfilled areas may vary over widelimits. The characteristics of size and shape are a result of the sizeand shape of the metal powders from which the structure was prepared, aswell as the compacting, sintering, working and annealing steps used inthe preparation.

The stress which the modified metals and alloys of the invention willsupport over a period of time at high service temperatures is at leasttwo to ten times larger than that of the unmodified metal and alloys.The resistance of the filler-modified metals and alloys to long-termdeformation under relatively low stress may be as much as ten thousandtimes better than the corresponding unmodified alloys. For instance, thestress for 100 hour rupture life of nickel when modified with oxidefiller as herein described, is improved at least twenty-fold whenmeasured at 1800 F. Not only are the products strong, but they areductile, readily machinable, and show considerable elongation understress, up to 90% of that of unmodified control.

By incorporating dispersed refractory particles into metal mix uresaccording to the invention the yield strength of the mixtures isquantitatively improved while the ductility of the mixtures, as measuredby the elongation, remains adequate for practical purposes. If Ym is theyield strength of the modified material at 0.2% off set and Ye is thecorresponding yield strength of the control, the following relationshipholds at temperatures in the range from 50 to of the melting point ofthe metal mixtures in degrees absolute:

A preferred class of the novel products consists of high-meltingcompositions, particularly those containing at least one of the metalsfrom the group consisting of iron, cobalt, nickel, molybdenum, andtungsten, together with a metal from the group consisting of chromium,titanium, and niobium. Of this group alloy compositions having a meltingpoint above 1200 C. are particularly advantageous.

A specifically preferred class of the novel products consists of alloyscontaining chromium. These alloys are particularly oxidation resistant.Because they have high-temperature strength by reason of the inclusionof the refractory oxide filler, they are useful at elevatedtemperatures, for instance, in the range of 1200 to 1800 F. and in somecases even higher. Stainless steel alloys are included in this preferredclass. They can be prepared from nickel-iron masterbatches containingrefractory oxide fillers such as thoria, by a process in which themasterbatch is blended with powdered chromium. Alternatively, amasterbatch of alumina particles in iron can be blended with chromium,nickel and iron powders, or a masterbatch of rare earth oxide particlescan be blended with iron-nickel powder. In a similar manner, one canmake other alloys of chromium such as Nichrome (80 Ni-20 Cr),iron-chromium, (73 Fe-27 Cr), and iron-nickel-cobalt-chromium alloyscontaining, for

=greater than 1.5:1

example, up to 30% chromium. Iron, nickel or cobalt base alloyscontaining from 10 to 25% chromium are a preferred group. Specifically,such alloys containing 90 to 50% of the sum of iron, cobalt and nickel,to 20% of the sum of molybdenum and tungsten and 0 to aluminum,titanium, manganese, silicon and niobium, along with to 25% chromium arean especially preferred species.

In the above-mentioned chromium alloys, and other high-temperaturealloys, it is preferred to use very stable refractory oxide fillers,that is, those with a high free energy of formation such as beryllia,calcia, thoria, and rare earth oxides, the filler having a free energyof formation, measured at 1000 C., in the range above 115 kilocaloriesper gram atom of oxygen in the oxide. Oxides having a free energy offormation at 1000 C. of up to 123 are presently available, and if morestable oxirlles could be prepared, they would be in the preferred c ass.

An especially preferred class of the novel products consists of alloyscontaining metals having high melting points, such as niobium, tantalum,molybdenum, or tungsten, or two or more of these metals. Molybdenum andtungsten have extremely high melting points and their presence raisesthe melting point of the alloy products formed. Since molybdenum ortungsten by themselves are not oxidation resistant, these metals are notordinarily used alone, but they are useful in alloys with other metals.Thus, especially preferred are alloys of these relatively high-meltingmetals with other metals such as nickel, iron, cobalt, chromium,titanium, zirconium, niobium, aluminum, and silicon. Thus, thispreferred group includes such alloys as high-molybdenum steel,nickel-molybdenum steel, molybdenum-iron-nickel alloys,tungsten-chromium and molybdenum-chromium alloys. Within this class,also, are alloys of molybdenum or tungsten with niobium or titanium, orwith both niobium and titanium. Molybdenum-titanium alloys, containingfrom 10 to 90% titanium, are included in this group, as aremolybdenum-niobium and tungsten-niobium alloys. The latter alloys can beconveniently prepared by the powder-blending process above described,using a molybdenum-filler masterbatch blended with niobium metal powder.

Still another preferred class of the novel products consists ofcompositions containing aluminum. Aluminum forms intermetalliccompounds, which are light in Weight and oxidation resistant. To make aproduct of this type one can, for example, add a lanthana-nickelmasterbatch to powdered aluminum, thereby obtainingaluminumnickel-lanthana compositions. Similarly, one can preparealuminum-copper alloys, aluminum-nickel-cobalt alloys, aluminum-ironalloys, and alloys containing both aluminum and molybdenum.

Compositions of this invention are especially useful for fabricationinto components which must maintain dimentional stability under heavystress at high temperatures, such as turbine blades. By hightemperatures is meant temperatures in the range from 0.5 to 0.8 timesthe melting temperature, in degrees absolute, of the metal in thecomposition.

Examples The invention will be better understood by reference to thefollowing illustrative examples.

Example 1 A solution of nickel nitrate was prepared by dissolving 4362grams of nickel nitrate hydrate Ni(NO .6H O in water and diluting thisto 5 liters. A thoria sol was prepared by dispersing calcined Th(C 0 inwater containing a trace of nitric acid. The thoria in this solconsisted of substantially discrete particles having an average diameterof about 5 to 10 millimicrons. This thoria was used as the source of thefiller material. A 288- gram portion of this colloidal aquasol (26% ThOwas diluted to 5 liters. To aheel containing 5 liters of water at roomtemperature, the solution of nickel nitrate, the diluted thoria sol, andammonium hydroxide-ammonium carbonate solution were added as separatesolutions simultaneously, and at uniform rates, while maintaining veryvigorous agitation. This was accomplished by using the apparatus shownin FIGURE 1. The valve to the filter was closed; the heel was chargedinto the tank; the pump was turned on; the feed streams were added asindicated. During the precipitation, the pH in the reactor wasmaintained at 7.5. A coating of nickel hydroxide-carbonate was thusdeposited around the thoria particles. The resulting mixture wasfiltered, and washed to remove the ammonium nitrate. The filter cake wasdried in an oven at 300 C.

The product obtained was pulverized with a hammermill to pass 325 mesh,placed in a furnace, FIGURE 2, and heated to a temperature of 500 C.Hydrogen was slowly passed over the powder at such a rate thatsufficient hydrogen was added to the nickel oxide to reduce it in aperiod of four hours. The flow of hydrogen was maintained at a steady,uniform rate during this reduction procedure for eight hours.Thereafter, the temperature was raised to 700 C. and the flow of dry,pure hydrogen was greatly increased, and finally the temperature wasraised to 1050 C. to complete the reduction, and sinter the reducedpowder.

The resulting powder had a surface area of 4 m. g. and a bulk density of2.3 grams per milliliter. The powder contained 10% ThO- by volume. Whencompacted and annealed, this powder had a Rockwell A hardness at 25 C.of '66.

Two parts of the thoria-nickel powder were blended with three parts ofcarbonyl nickel. This latter powder was about 5 to 9 microns in size.The carbonyl nickel, when compacted and annealed, had a Rockwell Ahardness of 26. The blended powder was then pressed hydraulically at 30tons per square inch to a billet 1 inch in diameter and 2 inches long.

The billet was next sintered in hydrogen (dew point 50 C.) for twentyhours at 550 C. and five hours at 1200 C. The nickel oxide content ofthe sintered billet was less than 0.01%

The sintered billet was then heated to 2200 F., dropped into a containerat 1100 F., and then extruded from the container through a die having a90 throat, to a 4-inch rod. Thus, sintering and hot-working were carriedout at temperatures high enough to achieve annealing too. The rod wastested as follows: Ultimate tensile strength at 1800 F. was 16,700p.s.i. and 0.2% offset yield strength was 16,500 p.s.i. The elongationWas 7%. A control sample of nickel made in the same way, but without thefiller, had a yield strength of 1,400 p.s.i. The improvement in yieldstrength at 1800 F. is thus a factor of twelve-fold for the fillednickel over the control sample.

The Rockwell A hardness of the sample was 51. This hardness did notchange on annealing for four hours at 2200 F.

Another improvement by which the product of this example ischaracterized is that of oxidation resistance. The oxidation rate at1800 F. in air, as measured by gain in weight, is slower for the 4%thoria-nickel sample of this example than for a wrought nickel,unmodified control. Specifically, the oxidation rate, as measured byWeight gain per unit surface area on heating in air at 2200 F. was aboutequivalent to the rate of oxidation of unmodified nickel at 1500" F.Specifically, after 4 cycles to temperature and a total time attemperature (2200 F.) of forty hours, the sample showed a total weightgain of only 2% The oxidation rate of the nickel thoria is about equalto the oxidation rate of wrought nichrome Ni-20Cr).

The stress-rupture properties also indicate stability. For example, asample of 4% thoria in nickel of this ex- 19 ample can withstand 7,000p.s.i. at 1800 F. for more than one hundred hours without rupture.

An electronmicrograph picture was prepared to show the distribution ofthoria in the thoria-nickel sample. The micrograph showed that therewere regions in which there was a homogeneous distribution of thoria inthe nickel. There were also regions in which no thoria Was seen. Theseareas correspond to the unmodified nickel which was used to blend withthe thoria-nickel. Ductility and machinability are better for sampleshaving such regions or islands of unmodified metal.

The electronmicrographs were prepared as follows: A fit-inch rod ofnickel containing dispersed thoria was cut and the cross section wasmounted in Bakelite and mechanically polished. The polished surface wascleaned and dried in ethyl alcohol. The samples were removed from theBakelite and placed in a high vacuum furnace and a vacuum of 10 mm. ofHg at 1000 C. was reached. After thermal etching for about three hours,the sample was removed and placed in a vacuum evaporator. Two carbonrods were brought together within the evaporator and current applieduntil sputtering occurred. A very thin film of carbon Was deposited uponthe etched surface as the sputtering occurred.

The carbon-covered surface was scribed into inch square with the use ofa sharp cutting blade.

Next the sample was placed in a culture dish containing a 1% solution ofbromine. The carbon squares were freed from the surface of the metal bychemical attack. They floated to the surface of the solution, werepicked up on electronrnicroscope screens (250- mesh S/S wire), andviewed in a Philips EM 100 threephase electronmicroscope. Alternativelysamples can be chemically etched, or viewed as polished.

The solution of bromine was used to remove the carbon because it wouldattack the base metal and not do damage to the oxide, or the carbonreplica.

All samples were photographed in the electronmicroscope at a filmmagnification of 1,250 and 5,000 respectively. Prints at 5,000X weremade from the 1,250 negative and at 20,000X from the 5,000X negative.

The presence of .grain boundaries (lines) in the 20,000 picture wasplainly observable. In the areas where filler particles were present,these grains averaged about 2 microns in size. The ThO particles wereabout 0.1 micron in size. In the areas where there was no filler, thegrain size was in the range of 50 microns, and greater, or abouttwenty-five-fold that of the filled grains.

There are several variables which affect the product quality as measuredin terms of yield strength at 1800 F. for products of the type describedabove, in Example 1. The following'illustrate this: (a) A masterbatch of30% was superior to one containing thoria. The former, for example, whenblended with nickel powder 1:1 gave a product having yield strength of24,000 p.s.i. and 1000-hour rupture strength of 8,000 p.s.i. at 1800F.(b) Preparations made from concentratcd nickel nitrate solutions aresomewhat better than those made from more dilute'nickel nitrate. (c) Ablending ratio of masterbatch to unfilled nickel in the range from 1:2to 1:1 was slightly superior to a blending ratio of either 7:3 or 114.

In addition, it is preferred to sinter the compacted billet in areducing atmosphere (such as hydrogen) rather than an inert atmosphere(such as argon), and to extrude at as low a temperature as possible.Extrusions have been made by heating the billet to 1800" F.

Example 2 This example is similar to Example 1, except that in thiscase, the final stages of the reduction and sintering were carried outat 950 C. The resulting nickel-thoria powder was blended with unmodifiednickel powder 20 (carbonyl grade, less than 325 mesh) to give productscontaining 2, 4 and 7% T110 From these and the starting material (10%ThO Ar-inch rods were prepared.

These rod products were characterized as follows: After heating to 1200"C, Rockwell A hardness for the sample containing 2% ThO was 44, for thesample containing 4% ThO was 51, for the sample containing 7% ThO was 58and for the sample containing 10% T110 was 66. The sample containing 4%ThO had a yield strength of 16,500 p.s.i. at 1500 F.; 13,000 p.s.i. at1800 F.; 18,300 p.s.i. at 1200 F. Thus, the decrease in yield strengthat 1800 F. is only 21% below the yield strength at 1500 F. This is amajor improvement in the characteristic behavior of the metalcomposition, since nickel alone and most of the nickel base alloys usedin high-temperature applications show a much larger percentage decreasein yield strength over this temperature-range. Pure nickel alone hasessentially no strength at 1800 F.

A comparison of the yield strength and elongation of these blendednickel structures with unmodified nickel (prepared from the sameunmodified carbonyl nickel powder which was used to blend with thethoria-nickel powder in the process and products of this example) andwith nickel-thoria of uniform structure (10% thoria) is shown below:

Ym=yield strength at 1800 F. of thoria-filled samples.

Yc=yield strength at 1800 F. of control, or unmodified nickel.

Percent elongation measured during yield strength test:

Eb=elongation of blended l'llCkGl'l10 112. product. Ec=Elongation ofcontrol or unmod fied n ckel. Eu=elongation of uniform 10% thoria innickel product.

It is evident from the above comparison that the products of theinvention were more ductile, i.e., they had more elongation at 1800F.than thoria-modified nickel in which the thoria was uniformlydistributed; this can be seen in the ratio Eb/Eu. In this particularseries, the elongation of the blended structures was approximately twotofour-fold that of the uniform structure. The elongations of the productsof the invention in this example were only reduced from about 0.5 to 0.8of that of the unmodified metal, i.e.,Eb/Ec was in that range. Also, theyield strength of all the thoriamodificd products was several-foldgreater than that of unmodified metal (ratio Ym/Yc).

Another improvement by which products of this example are characterizedis that of oxidation resistance. The oxidation rate at 1000 C. in air,as measured by gain in weight, is slower for the 4% thoria-nickel samplethan for a wrought nickel, unmodified control. Specifically theoxidation rate, as measured by weight gain per unit surface area onheating in air at 2200 F. is about equivalent to the rate of oxidationof unmodified nickel at 1500 F. In fact, the oxidation rate of thenickelthoria is about equal to the oxidation rate of wrought nichromeNi-20 Cr). The stress-rupture properties also indicate stability. Forexample, a sample of 4% thoria in nickel can withstand 11,000 p.s.i. at1500 F. for more than 1000 hours without rupture. At 1800 F. this samplecan support 4,500 p.s.i. for more than 1000 hours without rupture. Thestress-rupture curve measured at 1800 F. of this nickel-thoria samplehas a flatter slope than wrought Inconel or Hastelloy X, the latterbeing measured at a lower temperature, namely, 1500 F.

An electronmicrograph picture showing the distribution 20,000 p.s.i.

21 ofthoria in a thoria-nickel sample as prepared above (contaming 4volume percent thoria made by dilution of 4 parts of 10 volume percentthoria in nickel with pure nickel) shows that there are regions in whichthere is a homogeneous distribution of thoria in the nickel. There arealso regions in which no thoria can be seen. These areas correspond tothe unmodified nickel which was used to blend with the thoria-nickel.Ductility and machinability are better for samples having regions orislands of unmodified metal.

Example 3 A masterbatch of cobalt-thoria was prepared from (a) 4370grams Co(NO .6H O in liters H O, (b) 532 grams of 20.7% ThO sol dilutedto 4 liters, and (e) 25% (NH CO solution. This masterbatch was used fordiluting with unmodified cobalt powder according to the details ofExample 1.

It will be understood that the processes similar to those of Examples 1and 2 can be applied to other pure metals, including copper, iron andchromium.

Example 4 A nickel-thoria masterbatch, containing 30 volume percentthoria, was prepared according to the process of Example 1, the onlydiiferences being that three times as much thoria was used, and thefinal temperature of the reduction was 1l30 C. This powder was veryhard. In a similar manner, a nickel-zirconia powder, containing 0.5volume percent zirconia, was prepared. The zirconia aquasol used forthis latter preparation contained ZrO particles which were aboutmillimicrons in diameter. The zirconia sol at 10% solids had a relativeviscosity vs. water of 1.4. It was prepared by autoclaving 1 molarZr'O(NO solution at 200 C. and peptizing the resultant precipitate indistilled water. The nickel oxide zirconia composition was reduced at650 C. The resulting relatively soft powder had a Rockwell B hardness of67.

Equal parts of the two reduced nickel-filler powders were blended. Theblend was hydraulically pressed at 30 t.s.i. to form a billet 1 inch indiameter, and thereafter sintered in dry, pure hydrogen at 1200 C. Thisbillet was then extruded to a rod of fit-inch diameter. This rod had ayield strength at 1800" F. of greater than FIGURE 4 is a micrograph ofthe annealed metal product, showing the relative grain sizes in volumeswhere there is thoria as contrasted to zirconia. The grains in thevolumes containing the thoria were less than 1 micron in size, whereasthe grains in the volumes containing the zirconia were about 50 micronsin size. The ratio of grainsize in the two volumes was thus greater than50: 1.

In compositions of the type described in Example 4, the nickel-zirconiais regarded as the ductile phase. In such instances, it is preferredthat, as calculated from the expression where G is the grain size inmicrons, d is the particle diameter in microns, and f is the volumefraction of the filler, the grain size in the ductile phase beinggreater than 10 microns. More preferably, the calculated grain sizeshould be in the range from 40 to 150 microns. It will be understoodthat this grain size is calculated from the particle diameter of thefiller in the metal product, and not from the particle diameter of thefiller used in the preparation.

The average diameter of thoria particles which was processed at amaximum temperature of 1200 C. (during sintering) as in this example, is0.1 micron. Thus, if thoria is used in making the ductile phase, forcompositions of the type above described, a volume loading from 0.09 to0.3% is preferred. However, loadings somewhat above or below thispreferred range can be used to advantage.

hence a larger amount of filler may be used in the ductile phase.

Example 5 A nickel-thoria masterbatch powder containing 10 Volumepercent thoria was prepared as in Example 1. This powder was passedthrough a 325-mesh screen. It was then placed in a second furnace,FIGURE 3, and treated with hydrogen until the dew point of the efliuentgas was below 70 F. The furnace assembly was then transferred to a drybox, filled with argon. The powder was removed from the furnace in acompletely inert atmosphere, i.e., the oxygen level was about 5 partsper million. Nitrogen and water vapor in the dry box were alsocorrespondingly low.

While in the dry box, the powder was blended with nickel and chromiumpowders as follows: 44 parts of nickel-thoria by weight with 40 partsnickel and 20 parts chromium. The oxygen analysis of the nickel powderwas 0.00%. The oxygen analysis of the chromium powder was less than0.01%. The nickel powder was 10 microns in size and the chromium powderhad a dendritic structure and was less than mesh, in the largestdimension. Both of the unmodified metal powders were softer and moreductile than the thoria-nickel powder.

The blended powder was then hot pressed to a billet of 100% density, thetemperature of the containerand powder during pressing being 1100 F.Only after this pressing was the billet exposed to air (oxygen). Thebillet was 1 inch in, diameter and 2 inches long. It was extruded toA-inch rod, as in Example 1.

After extrusion, the rod was annealed in pure, dry hydrogen at 1325 C.for fifty hours. Alternatively, the billet can be sintered prior toextrusion. The extruded, annealed rod can support 8000 p.s.i. for over100 hours at 1800 F. The extruded, annealed rod contained grains incontact with thoria particles which were about 2 microns in size andabout an equal volume of grains, not in contact with thoria, which wereabout 100 microns in size.

The stress-rupture plot at 1800 F. of the product of Example 5 isflatter than the stress-rupture plot of the nickel-thoria product ofExample '1.

Example 5 illustrates the care which was taken to exclude oxygencontamination of the powders used to prepare the solid, wrought metalproduct. It is preferred that similar care be exercised in the case ofcompositions including alloys of niobium, titanium, silicon, aluminum,and other similar metals which form stable oxides.

Instead of hot pressing, the blended powder can be hydrostaticallycompaced, as, for example, with pressures in the range from 20,000 to200,000 p.s.i. In this case the compact should preferably only beexposed to air after sintering. In this case, sintering at 1325 C. canbe done prior to extrusion and the annealing after extrusion eliminated.Alternatively, the mixed powders can be canned, extruded, and finallyannealed.

Example 6 This example describes the utility of a chromium-thoriam-asterbatch in making compositions of the invention.

The reactor used to deposit the chromium oxycarbonate on the colloidalthoria consisted of a stainless-steel tank with a conical bottom, shownin FIGURE 1. The bottom of the tank was attached to a stainless-steelcirculating line, to which there were attached three inlet pipes throughTts. The circulating line passed through a centrifugal pump and thencereturned to the tank.

Initially, the tank was charged with 10 gallons of water, which wasabout /5 of the capacity of the tank. Three feed solutions were preparedas follows: (a) 40.6 pounds of chromium nitrate, Or(NO .9H O, dissolvedin 5 gal- 3, 1 aoAas lons of water, (b) 30 pounds of ammonium carbonate:dissolved in gallons of water, and (c) 20 liters of a colloidal aquasolcontaining 3% thoria, in the form of 5 to millimicron particles, dilutedto 5 gallons with water.. These three feed solutions were metered inthrough call-- brated liquid flow-meters at equal rates into thecirculating stream, which initially consisted of water. The pH of theslurry in the tank was maintained between 7.0 and 8.0 during the run,and was 7.6 at the end of the run. The time of addition of the reactantswas 40 minutes, the reactants being added at room temperature.

The resulting slurry contained precipitated particles which consisted ofhydrous, chromium oxycarbonate and colloidal thoria. This precipitatewas filtered, and washed with water to remove most of the soluble salt.It was then dried for forty hours at 250 C. and micropulverized, to givea product which passed 100 mesh. The analysis of the product was 18.1%ThO and 78.9% Cr O X-ray line-broadening studies showed that the chromiacoating consisting of crystallites about 1 micron in size. The thoriaparticles entrapped in this chromia coating had an average size of about40 millimicrons.

The product thus obtained was reduced by treating it with pure, dryhydrogen at 1300 C., until the dew point of efiluent hydrogen was below-70 C. The reduced, hard, Cr-Th0 powder was then blended with arelatively soft nickel powder containing 0.3 volume percent thoria insuch a ratio as to yield an alloy of 80% nickel, chromium. The blendedpowder was hot pressed to a dense billet, completely free of voids.During the entire handling of the powders, oxygen, nitrogen and watervapor were completely excluded.

Thebillet was sintered at 1325 C. for forty hours in pure, dry hydrogen,after which time the nickel and chromium were homogenized. The billetwas then ex truded from l-inch to A-inch diameter. The product had ayield strength at 1800 F. of 15,000 p.s.i. and an elongation at thattemperature of 23%. Thus, the yield strength at 1800 F. was double thatof a control prepared from metals fabricated as indicated but withoutoxide filler, and elongation was 77% that of control. The structure ofthe annealed product consisted of grains about 2 microns in size in 20%of the metal volume and grains about 80 microns in size in the remainingvolumes.

Examnple 7 Thisex-ample shows an alternative process for preparingproducts of the invention. In this process, the powders are blendedprior to reduction.

A nickel oxide-alumina was prepared according to the processof Example1, by using an alumina sol prepared by dispersing Alon C alumina powderin a dilute acid solution. :In a similar manner, a nickel-oxide powderwas prepared, without any filler. Both powders were dried at 450 C. andmicro-pulverized to .100 mesh.

Equal weights of the two powders were blended. The blend was thenreduced in extra dry, pure'hydrogen at 550 C. This powder was extremelypyrophoric, hence it was not exposed to oxygen.

The reduced powder was handled in an inert atmosphere (argon) in whichit was hot pressed, and the pressed billet was then sintered in hydrogenat 1250 C. for twenty-four hours. The ,sintered billet ,had a densityequivalentto theoretical. Only after sintering was it exposed to air.

This. billet was extruded from 1 inch to inch, whereupon anickel-alumina product containing 6 volume percent Al O having superiorhigh-temperature properties was produced.

Example 8 A masterbatch of Cu-% (volume) A1 0 was prepared from Cu(NOsolution and A1 0 aquasol, using the precipitation technique of theprevious examples. An

24 improvedmonel was prepared by blending 52.5 parts by weight of thismasterbatch (-325 mesh) with 122.5 parts by weight of pure nickel powder(8 micron).

The blend was hot pressed at 600 C. and 25 t.s.i. to a billetof greaterthan 98% density. This billet was sintered at 1100 C. and extruded. Theproduct had improved resistance to creep at 1700 F.

By blending the above masterbatch with zinc powder, one can prepareimproved brass. Other copper alloys, including bronzes, can be similarlyprepared.

Example 9 This example describes a modified stainless-steel compositioncontaining 4% thoria by volume dispersed therein, the thoria being inthe form of colloidal particles. The process consists of blendingnickel-iron-thoria masterbatch powder with chromium powder, densifying,and homogenizing.

A thoria concentrate in nickel-iron was prepared as :follows: A depositof iron-nickel, hydrous oxycarbonate was formed on a colloidal thoriafiller in a reactor consisting of a stainless-steel tank with a conicalbottom, FIGURE 1. The bottom of the tank was attached to stainless-steelpiping, to which were attached three inlet tubes through Ts, thiscirculating line then passed through a centrifugal pump of 20 g.p.m.capacity, thence through a return line to the tank.

Initially, the tank was charged with 2 gallons of water. Equal volumesof three solutions containing the desired quantities of reagents werethen added into the middle of the flowing stream through the T tubes.These solutions were added at uniform, equivalent rates over a period ofabout one-half hour. Through the first T was added a solution of ironnitrate-nickel nitrate prepared by dissolving 2190 grams Fe(NO .9H O and169 grams Ni(NO .6H O in water and diluting to 3.7 liters. Through thesecond T was added 3.7 liters of 3.4 molar (NH CO and through the third3.7 liters of thoria sol made by diluting 60 grams of 36% ThO aquasolwith water. The thoria aquasol was highly fluid and contained particlesin-the 5 to 10 millimicron size range.

The solutions were added into the reactor simultaneously while the pumpwas in operation. The rate of addition controlled uniformly byflowmeters. The pH of the solution in the tank was taken at frequenttime intervals to insure proper operating, the pH remaining essentiallyconstant during the run and the final pH being 7.7. The slurry wascirculated for a few minutes after the addition of the reagents had beencompleted, and then the solution was pumped into a filter. Theprecipitate was filtered, washed with water, and dried at a temperatureof about 300 C. for twenty-four hours.

The dried product was then pulverized by grinding in a hammermill, andscreened to pass 325 mesh.

The pulverized material was placed in a furnace at a temperature ofabout C. and a mixture of argon and hydrogen was slowly passed over it.This gas stream had previously been carefully freed of oxygen, nitrogen,and moisture. The temperature in the furnace was slowly raised over aperiod of an hour. The flow of hydrogen was then gradually increased,and the temperature in the furnace was gradually raised until 600 C. wasreached, whereupon a large excess of hydrogen was passed over the samplein order to complete the reduction. In this way, the sample wascompletely reduced.

Finally, the temperature was raised to 850 C., while continuing to passhydrogen over the sample. In this way, an iron-nickel powder wasproduced containing 5 volume percent thoria, having a surface area lessthan 2 m. /g., and being essentially oxygen free, i.e., having an oxygencontent, exclusive of the ThO filler, of less than 0.01%.

This powder was used in making a stainless-steel of exceptionalhigh-temperature properties, as follows: 83 parts by weight of themodified iron-nickel powder (less than 200 mesh) was blended with 18parts of less than 325-mesh ductile chromium powder. The thoria-metalconcentrate was protected from the air, as in previous examples, toprevent reoxidation and the chromium powder used was oxygen free. Theblend was hot pressed at 60 t.s.i. into a dense billet 1 inch indiameter and about 2 inches long. The billet was then sintered inextremely dry hydrogen at 1300 C. for eighteen hours, the temperaturebeing raised to the maximum level over a period of 6 hours in thisprocess. The hydrogen used had previously been freed of oxygen, water,and nitrogen by passing it through sulfuric acid, then through acommercial dehydrating agent, then through magnesium perchlorate at 95C., through a Molecular Sieve, also at 95 C., and finally over Ti-Zr-Crchips at 800 C. The Molecular Sieve used was type 4A, /s-inch pellets,available from Linde Air Products Division of Union Carbide. Aftersintering, the billet was exposed to air.

The billet was next machined to a diameter of 0.93 inch and hot extrudedrapidly to a Ar-inch rod. This rod was a stainless-steel having aRockwell C hardness of 30, which did not change even after long aging at1300* C. The resulting product was a dispersion of about 150 millimicronTh0 particles in a stainless-steel matrix.

There were some islands of metal free from thoriar Grains in theseislands were greater than 50 microns in size. Examination of thestructure showed that the material was completely austenitic(non-magnetic), and that the grain size was about 2 microns in theregions in which filler was present. The extruded rod had a 0.2% offsetyield strength of 24,300 p.s.i. at 1500 F.

Example 10 This example describes a masterbatch of chromium and tungstencontaining oxide filler. This masterbatch represents a preferredcomposition, for blending with other powdered metals for preparingproducts of the invention. Thus, it is particularly useful as alloyingagents to add to other powdered metals in preparing super alloys, as,for example, for preparing improved types of S-495, S-588, ATV-3, S-497,S-590, SF816, Refractaloy 70, Refract-alloy 80, M-203, M-204, M205, 25Ni, Hastelloy C., Thetalloy C., HS23, HS-25, HS31, HS36, X-50, WF-31,I-336, HE-1049 (see Appendix 11 Report on the Elevated-TemperatureProperties of Selected Super- Strength Alloys published by the AmericanSociety for Testing Materials, STP No. 160).

This reinforced chromium-tungsten alloy is prepared from four startingsolutions: (a) 185 grams (NH W O .4H O

dissolved in 10 liters H O, (b) 780 grams CI'(NO3)3.9H2O

in liters H O, (c) 6 grams beryllia as 100 rnillimicron colloidalparticles (see Weiser, Inorganic Colloid Chemistry, Volume II, page 177,I. Wiley and Sons, 1935) in 5 liters H 0, and (d) 5 liters of (NI-I COThese solutions are fed into 2 gallons of water in a reactor tankthrough four T tubes.

The precipitate is filtered, washed with a dilute (0.01%) solution ofammonium carbonate, dried and pulverized. The pulverized powder is thenreduced as follows: It is placed in a gas-tight furnace. The furnace isthen heated to 250 C. and evacuated. The temperature in the furnace israised to 500 C. and maintained there for six hours while an excess ofextremely pure, dry hydrogen is passed over the powder. Thereafter, thetemperature in the furnace is increased at the rate of 25 per hour until1100 C. is reached. Then the temperature is increased to 1250 C. andheld there until the dew point of the effluent hydrogen is below 60 C.

During the reduction the following precautions are observed: Oxygen andnitrogen and their compounds are completely eliminated from the hydrogenfeed. The water vapor produced during the reduction is swept out of thefurnace by passing hydrogen over the sample at a high flow rate. Thetemperature in the reducing zone is kept constant, i.e., no variationfrom one place to another of more than 20.

The resulting chromium-tungsten powder, modified with 10 volume percentberyllia is useful as an alloying agent for blending with other powderedmetals, including nickel, cobalt, iron, titanium, aluminum, niobium,chromium, tungsten and combinations of these.

The powder is characterized by having a uniform distribution of 200millimicrons beryllia particles throughout the metal. Thus, the averageinterparticle distance is 140 millimicrons. The grain size of the metalis small, of the order of 1 to 5 microns. This grain size does notchange appreciably in size, even on prolonged aging at 1200 C.

By increasing the amount of beryllia, one can prepare metal productscontaining 20, 30, or more percent of oxide. By changing the ratio ofCr(NO .9H O and (NH W O .4H O, one can vary the ratio of Cr to W in theproduct. (Such a change may require an adjustment in the amount of (NHCO used.) By substituting (NHQ M O AH O for the tungsten compound, onecan prepare alloys of chromium and molybdenum. Further, one cansubstitute colloidal T110 A1 0 La O 0 Ce O Sm O S0 0 other of the rareearth oxides, including mixtures of rare earth oxides, in place of thecolloidal beryllia, and thus prepare Cr-W alloys containing any of theseother oxides.

In such alloys it is preferred to use oxides in the form of particlesless than 250 millimicrons in size, on the average, and the oxides whichhave a high free energy of formation. Also, it is preferred to usereduction temperatures as low as is possible, commensurate withcompletely reducing the Cr to Cr.

Using the technique above described, a Cr-W-Al O masterbatch containing1.37 parts Cr and 1 part W by weight and 15% A1 0 by volume is prepared.An alloy is prepared by blending 38 parts of this masterbatch with 10parts of nickel and 51 parts of cobalt. All powders used are oxygenfree, except for oxygen in the A1 0 filler. The masterbatch powder was325 mesh. The nickel and cobalt powders are hot pressed at 25 t.s.i. toa billet of 98% density. This billet is then hot-worked under conditionsof plastic flow.

Example 11 A sample of iron powder containing 10 volume percent Al O wasprepared similar to the process of Example 1, using a dispersion of A1 0in dilute HNO in place of the ThO sol and Fe(NO solution in place ofNi(NO .6H O. The A1 0 dispersion was prepared by slurrying a commercialA1 0 powder, Alon C, in very dilute nitric acid, colloid milling anddiscarding the fraction which settled in a 10-inch container over aperiod of twenty-four hours.

Example 12 An improved nickel-molybdenum alloy was prepared by blendingnickel and iron powder with a masterbatch of molybdenum-thoria.

The masterbatch was prepared by precipitating molybdenum hydroxide inthe presence of colloidal thoria and reducing the precipitate withhydrogen. This was done by adding (a) 1 liter of 2M MoCl (b) 1 liter ofThO sol (95 grams 22% ThO sol, prepared by calcining Th(C O diluted to 1liter) and 0.85 liter M NH OI-I solution to a heel of 1 liter H O.

The precipitate was dried, heated at 450 0., ground to 325 mesh,reduced, and finally sintered in hydrogen at ll50 C.

An alloy was prepared by blending 22 parts of this Mo-ThO masterbatchwith 60 parts Ni and parts Fe powder. The latter powders were less than10 microns average particle size. All powders contained less than 0.01%oxygen, exclusive of the thoria.

Another way of preparing alloys of the type described in this example isto prepare a masterbatch powder of Mo-Fc-Ni-filler and blend this with ametal powder consisting of Mo-Fe-Ni. In this case, the ratio of Mo to Feto Ni in the masterbatch would be the same as in the unmodified orductile metal powder. This procedure has the advantage that homogenizingof the metal fraction is accomplished, for example, by chemical means.Thus, lengthy heat treatments of the blended masses can be eliminated.

'The blended powder was hot pressed in an inert atmosphere (pure, dryargon). It is then ready to be rolled, heat treated at an intermediatetemperature of about 900 to 1000 C. and rerolled until the reduction inthickness of the initial piece of metal is twenty-fold. The metalliccomponents are finally homogenized by a long anneal at 1325 C.

In a similar way, one can prepare molybdenum-thoria masterbatches, andfrom these molybdenum-titanium and molybdenum-niobium alloys containingfillers. In the case of these higher melting alloys, the final heattreatment is done at a correspondingly higher temperature.

Eldample 13 This example describes a cobalt-nickel composition modifiedwith 2.5 volume percent thoria, said composition being useful inpreparing an improved high-temperature alloy.

The preparation followed the general details as outlined in Example 1except for the following: The feed solutions consisted of: (a) 1125grams Co(NO .6H O and 2470 grams Ni(NO .6H O in 5 liters H O, (b) 57.4grams ThO sol containing 36.4% solids diluted to 5 liters, and (c) 1900grams (NI-I CO dissolved in H 0 and diluted to 5 liters. Reduction wascarried out at 500 to 600 C. and sintering in hydrogen for one-half hourat 850 C.

This powder (325 mesh) of modified nickel-cobalt was then useful forblending with other metal powders.

Example 14 This example describes the preparation of a nickelzirconia(volume) masterbatch which was used for diluting with unmodified nickelpowder. This material differs from preparations described in previousexamples in that the nickel hydroxycarbonate was first precipitated,filtered, washed, and then reslurried, whereupon colloidal zirconiaaquasol was added to yield a zirconia impregnated nickelhydroxycarbonate. After subsequent filtering, drying, reducing, andsintering, the resulting nickelzirconia was blended 1:1 by weight withunmodified nickel powder according to the details of Example 1, to yielda Ni-15% zirconia product.

A solution of nickelous nitrate hydrate was prepared by dissolving 4370grams Ni(NO .6H O in water and diluting this to 5 liters. To a heelcontaining 5 liters of water at room temperature, this solution wasadded simultaneously and at a uniform rate with a 25% ammonium carbonatesolution while maintaining vigorous 28 agitation. This was carried outin the apparatus shown in FIGURE 1. During the precipitation, the pH inthe reactor was maintained at 7.1. The resulting nickel hydroxycarbonatewas filtered and washed repeatedly with water to remove ammonium salts.

One third of the resulting wet cake was reslurried in 3 liters of watersolution, the nickel hydroxycarbonate maintained in suspension byvigorous agitation. To this suspension was added over a thirty-minuteperiod three liters of water solution containing 78 grams of dispersedcolloidal zirconia, while maintaining vigorous recycling in theapparatus of FIGURE 1. The colloidal zirconia consisted of particles offrom 5 to 20 millimicron average particle diameter prepared according toprocedures given in copending US. patent application Serial No. 625 188filed November 29, 1956, now Patent No. 2,984,628 by Guy B. Alexanderand John Bugosh. The resulting zirconia-nickel hydroxycarbonateproduct'was filtered and dried in an oven at 260 C. The dried materialwas reduced with hydrogen while maintained at 1025 C. to yield a Ni-30%zirconia (volume) product.

One part of the nickel-zirconia powder was blended with one part of acarbonyl nickel, the blended powders compacted according to theprocedure of Example 1, and the resulting billet sintered in dryhydrogen for eight hours at 900 C. The sintered billet was then extrudedat 1800" F. and the resulting Ni-15% zirconia product had a tensilestrength of 26,000 p.s.i. measured at 1500 F.

Example 15 A nickel-rare earth oxide powder containing 10 volume percentdidymium oxide was prepared as in Example 1. A didymia sol was preparedby reaction of Na O with anhydrous didymium chloride in molten NaCl-KCleutectic (50:50 mole percent) at 700 C. The didymia was isolated fromthe solidified salt cake by leaching away of the salts with water,followed by repeated washing of the didymia until a sol was obtained.The didymia in this sol consisted of substantially discrete particleshaving an average diameter of about millimicrons. This didymia served asthe source of the filler material. A 932-gram portion of this colloidalaquasol containing 70 grams didymia was diluted to 5 liters. To a heelcontaining 5 liters of water at room temperature, the diluted didymiasol, a solution of 4370 grams Ni(NO .6H O in 5 liters of water, and 25%ammonium carbonate solution were added simultaneously as separatesolutions and at uniform rates, while maintaining vigorous agitation byusing the apparatus of FIGURE 1. The pH in the reactor was maintained at7.1 during the precipitation.

After filtration and drying of the cake, the didymianickelhydroxycarbonate was reduced at 950 C. then blended with unmodifiednickel powder, and fabricated into a billet as in Example 1. Aftersintering and hot extruding the billet, the Ni-4% didymia product had ameasured tensile strength of 16,000 p.s.i. at 1800 F.

Example 16 A nickel-didymia masterbatch containing 30 volume percentdidymia oxide was prepared according to the processes of Example 15, thedifferences being that three times as much didymia was used, and thefinal temperature of the reduction was 1100 C. The didymia sol used wasobtained by peptizing calcined didymium oxalate in weakly acidicsolution. This masterbatch was used for diluting with unmodified nickelpowder. The blend ratio was 1 part of masterbatch and 1 part ofunmodified nickel powder.

This application is a continuation in part of our copending U.S.application Serial No. 694,086, filed November 4, 1957, as acontinuation in part of our then 00- pending but now abandoned US.application Serial No. 657,507, filed May 7, 1957.

We claim:

1. In a process for producing compositions in which a particulaterefractory is dispersed in a metal the steps comprising (a) depositing ahydrous, oxygen-containing compound of a metal having an oxide with a AFat 27 C. of from 30 to 105 kcaL/gram atom of oxygen, together withsubstantially discrete, submicron particles of a refractory oxide havinga melting point above 1000" C. and a AF at 1000 C. above 60 kcal./ gramatom of oxygen, (b) thereafter reducing the hydrous, oxygen-containingcompound to the corresponding metal while maintaining a temperaturethroughout the entire mass below the sintering temperature of the metalthe reduction being continued until the oxygen content of the mass,exclusive of oxygen in the refractory oxide, is below 2% by Weight,whereby a powdered product is obtained, sintering the powdered productat a temperature below the melting point of the metal until its particlesize is in the range of l to 500 microns, and (d) mixing the sinteredpowder with up to 10 times its volume of a ductile powdered metal.

2. A process of claim 1 wherein the volume ratio of sintered powder topowdered metal is from 0.321 to 2.5: 1.

3. In a process for producing compositions in which a particulaterefractory is dispersed in a metal the steps comprising (a) depositing ahydrous, oxygen-containing compound of a metal having an oxide with a AFat 27 C. of from 30 to 105 kcaL/gram atom of oxygen, together withsubstantially discrete, submicron particles of a refractory oxide havinga melting point above 1000 C. and a AF at 1000 C. above 60 kcal./ gramatom of oxygen, (11) thereafter reducing the hydrous, oxygen-containingcompound to the corresponding metal while maintaining a temperaturethroughout the entire mass below the sintering temperature of the metalthe reduction being continued until the oxygen content of the mass,exclusive of oxygen in the refractory oxide, is below 2% by weight,whereby a powdered product is obtained, (0) sintering the powderedproduct at a temperature below the melting point of the metal until itsparticle size is in the range of 1 to 500 microns, (d) mixing thesintered powder with a ductile powdered metal, the volume ratio ofsintered powder to powdered metal being from 0.3:1 to 25:1, (e)sintering the mixture to form a metal product having a density of from60 to 100% of its absolute density, and (f) hot-working the resultingproduct.

4. A process for making metal parts having improved high-temperatureproperties, the process comprising blending a ductile powdered metalwith a powdered metal composition having uniformly dispersed thereinparticles, having an average dimension of 5 to 250 millimicrons, of anoxide having a free energy of formation at 1000 C. above 60 kilocaloriesper gram atom of oxygen and having a melting point above 1000" C., theproportions being from 0.1 to parts by volume of ductile, powdered metalper part of the oxide-containing composition, compacting the blendedpowders, sintering the compact, and hot-working it and forming it to theshape of the part to be made.

5. A process for making metal parts having improved high-temperatureproperties, the process comprising blending a ductile powdered metalwith a powdered metal composition having uniformly dispersed thereinparticles, having an average dimension of 5 to 250 millimicrons, of anoxide having a free energy of formation at 1000 C. above 60 kilocaloriesper gram atom of oxygen and having a melting point above 1000 C., theproportions being from 0.1 to 10 parts by volume of metal powder perpart of the oxide-containing compositions, and compacting the mixture toa mass having an apparent density from 90 to 100% of absolute density.

6.'A pulverulent metal composition comprising a dispersion of aplurality of submicron particles of a refractory oxide having a meltingpoint above 1000 C. and a AF at 1000" C. above 60 kilocalories per gramatom of oxygen in the oxide, in powder particles of a metal having anoxide with a AF at 27 C. of from 30 to 105 kcaL/ gram atom of oxygen,said dispersion having a surface area less than 10 square meters pergram and being mixed with up to 10 times its volume of a ductilepowdered metal containing substantially no refractory particles, therefractory oxide and the metals being so selected that the AF of saidoxide is at least 52+0.0l6M, where M isthe melting point of the mixtureof metals in the final composition, in degrees Kelvin.

l 7. A pulverulent metal composition comprising a dispersion of aplurality of submicron particles of a refractory oxide having a meltingpoint above 1000 C. and a AF at 1000 C. above 60 kilocalories per gramatom of oxygen in the oxide, in powder particles of a metal which has anoxide with a AF at 27 C. of from 30 to 105 kcal./ gram atom of oxygen,said dispersion having a surface area less than 10 square meters pergram and the average grain diameter of the metal in said dispersionbeing less than 5 microns, the powder dispersion being mixed with aductile powdered metal which after annealing has an average graindiameter greater than 10 microns, the volume ratio of metal in theinitial dispersion to ductile, powdered metal being from 0.321 to 2.5:1, the refractory oxide and the metals being so selected that the AF ofsaid refractory oxide is at least 52+0.0l6M, Where M is the meltingpoint of the mixture of metals in the final composition, in degreesKelvin.

8. A composition comprising powder particles of a metal selected fromthe group consisting of iron, cobalt, and nickel having uniformlydispersed in each powder particle a plurality of particles, having anaverage dimension of 5 to 250 millimicrons, of an oxide having a freeenergy of formation at 1000 C. above 60 to 150 kilocalories per gramatom of oxygen and having a melting point above 1000 C., the compositionhaving a surface area less than 10 square meters per gram, and havingmixed therewith up to 10 times its volume of ductile, powdered metal.

9. A solid metal composition in which there are (01) volumes of a metalconsisting of grains smaller than 5 microns in average diameter andhaving substantially uniformly dispersed therein a plurality ofdiscrete, submicron particles of a refractory metal oxide having a AF at1000 C. of more than 60 kcaL/gram atom of oxygen, the metal being of theclass which have oxides with a AF at 27 C. of 30 to 105 kcaL/gram atomof oxygen, and (b) intermingled with said first volumes, other volumesof metal having grains larger than 10 microns in average diameter andwhich are substantially free of said refractory metal oxide particles,the proportion of oxide-free to oxidefilled volumes being up to 10:1,the entire metal composition having an apparent density which is from toof the absolute density, and the refractory oxide and metal being soselected that the AF of the refractory oxide is at least 52+0.0l6M,where M is the melting point, in degrees Kelvin, of the metal in thecomposition.

10. A solid, annealed metal composition in which there are (a) volumesof a metal of the class which have oxides with a AF at 27 C. of 30 tokcal./gram atom of oxygen, the grains of said metal being smaller than 5microns in average diameter and having uniformly dispersed thereindiscrete, submicron particles of a refractory metal oxide having a AF at1000 C. which is more than 60 kcaL/ gram atom of oxygen and at least thevalue calculated from the expression 52+0.0l6M, where M is the meltingpoint of the metal in degrees Kelvin, and (b) other volumes of the samemetal in which the grains are larger than 10 microns in average diameterand are not in contact with said refractory oxide particles, theproportion of oxide-free to oxide-filled volumes being up to 10:1, theaverage grain size in the volumes (b) being at least two-fold theaverage grain size in volume (a).

11. A solid metal composition in which there are (a) volumes of a metal,the grains of which are less than 5 microns in average diameter, saidgrains having uniformly dispersed therein discrete, submicron-sizedparticles of a refractory metal oxide having a AF at 1000 C. which is

1. IN A PROCESS FOR PRODUCING COMPOSITIONS IN WHICH A PARTICULATEREFRACTORY IS DISPERSED IN A METAL THE STEPS COMPRISING (A) DEPOSITING AHYDROUS, OXYGEN-CONTAINING COMPOUND OF A METAL HAVING AN OXIDE WITH A $FAT 27* C. OF FROM 30 TO 105 KCAL./GRAM ATOM OF OXYGEN, TOGETHER WITHSUBSTANTIALLY DESCRETE, SUBMICRON PARTICLES OF A REFRACTORY OXIDE HAVINGA MELTING POINT ABOVE 1000*C. AND A $F AT 1000*C. ABOVE 60 KCAL./GRAMATOM OF OXYGEN, (B) THEREAFTER REDUCING THE HYDROUS, OXYGEN-CONTAININGCOMPOUND TO THE CORRESPONDING METAL WHILE MAINTAINING A TEMPERATURETHROUGHOUT THE ENTIRE MASS BELOW THE SINTERING TEMPERATURE OF THE METALTHE REDUCTION BEING CONTINUED UNTIL THE OXYGEN CONTENT OF THE MASS,EXCLUSIVE OF OXYGEN IN THE REFRACTORY OXIDE, IS BELOW 2% BY WEIGHT,WHEREBY A POWERED PRODUCT IS OBTAINED, (C) SINTERING THE POWDEREDPRODUCT AT A TEMPERATURE BELOW THE MELTING POINT OF THE METAL UNTIL ITSPARTICLE SIZE IS IN THE RANGE OF 1 TO 500 MICRONS, AND (D) MIXING THESINTERED POWDER WITH UP TO 10 TIMES ITS VOLUME OF A DUCTILE POWDEREDMETAL.